This article is the second in a series on the
Commission's theme, "Habitat." Clean watery habitat is one thing. But how much water fish need is something else. The
author explains the Commission's role in stream flow protection and how fish habitat and flow are connected.

"Habitat" is the place or type of site where a plant or
animal is naturally found. That's how my dictionary defines this term. In its most basic sense, the habitat for fish is
the water. That is, it's in the water where fish are naturally found. Sure, fish are typically most frequently found in
specific areas in the water, like under an undercut bank or root wad, in weed beds near shore, or under or behind the
larger rocks. But it is ultimately in the water that fish live and move about. Take away this most vital medium of life,
and you will not have fish.

Why spend so much time on the obvious point that fish need
water to live? A closely related concern that often arises for Commission biologists is not so obvious-how much water do
fish need?

The best answer to this question is: It depends. For
example, during droughts, the answer may be that they need it all. During floods, it may possibly be that they need
less. And the rest of the time? I think you may be starting to get the picture.

West Fairview Access at the confluence of the
Susquehanna River and Conodoguinet Creek, Cumberland County. This photograph shows the very low water conditions in the
fall of 1989.

This photograph, taken from the same vantage point as
the photograph above, shows the access during the flood conditions in 1993. Droughts and floods are natural
conditions. Droughts create habitat conditions that can favor some fish species. In addition, streams need floods.
Floods scour the stream and redistribute its bed load. photos-Art Michaels

Natural flow regime

To address this matter of how much water a fish needs,
let's start at the beginning with an introduction of the term "natural flow regime." The natural flow regime is a
description of how flow changes on a particular stream over time. Flows change almost daily on most streams. Over the
course of time, they may range over several orders of magnitude. For example, the average flow on the Susquehanna River
at Harrisburg has been continuously monitored by the United States Geological Survey since 1891. That flow ranges from
7.5 billion gallons per day (bgd) in September to 48.0 bgd in March (see Figure 1). The lowest flow ever recorded at
Harrisburg was 1.1 bgd on November 29, 1930. The highest flow ever recorded was 616.3 bgd on June 24, 1972, during the
Hurricane Agnes flood. Expressed another way, there were over seven million gallons of water passing Harrisburg every
second on that day.

When most people think of flowing water, they probably view
both floods and droughts as "bad," and most other flows as either OK or not worth thinking about at all. From a fish's
perspective, all flows are important. Fish need floods, fish need droughts and fish need the flows in between. They are
adapted to do just fine under these naturally changing conditions. However, when the natural flow regime is altered in
some way by people, fish can get into trouble.

Floods, droughts and more

The reason why fish need floods is that streams need
floods. Streams are beautifully designed to transport bed load continually. "Bed load" is the silt, sand, gravel, cobble
and boulders that comprise the bottom of the stream. The shape of the stream and the composition of the bed load are
critical components of fish habitat. Diverse substrate types typically favor diverse fish assemblages. The process of
maintaining the substrate composition and the shape of stream channels can best be described as a dynamic equilibrium.
This means that the stream is constantly changing at the fine scale, yet always staying the same at the large scale. A
flood is the most important phenomenon that occurs on streams to redistribute the substrate and keep the bed load moving
through the system. The need for floods was highlighted at the national level in 1996 when experiments were initiated to
mimic a flood in the Grand Canyon. This natural process was interrupted since the Glen Canyon Dam was constructed on the
Colorado River in 1965. Bottom substrates, particularly sand, were redistributed during this event, and the formation of
sand bars, which are particularly important to that river's ecology, was significantly enhanced.

Droughts are also natural phenomena on streams and rivers.
During droughts, the instream habitat conditions can actually be favored for some fish species, such as certain minnows
and darters, and fry and fingerlings of larger species. Drought conditions allow these fish to compete with other fish,
such as larger predators, which may be favored at higher flows. The result is a more robust and diverse fish community.
An example of this phenomenon, which has been demonstrated in studies across Pennsylvania, is that smallmouth bass
reproduction can be suppressed during particularly high flow conditions during their spawning period in May and early
June, and enhanced with relatively low, stable flows during this same period.

Measuring the effect of flow on fish habitat

The most common method used today in Pennsylvania to
determine "how much water the fish need" is called the Instream Flow Incremental Methodology (IFIM). This methodology
was designed by a multidisciplinary United States Fish and Wildlife Service team of biologists, hydrologists, engineers
and computer scientists working together in Colorado in the 1980s. Development of the methodology in the West makes
sense when one considers the infamous "water wars" for which this relatively dry region of the country is known.

The methodology has been refined in many ways over the
years, but in essence it involves the development of several computer models that are used to define the relationship
between flow and fish habitat. Four basic types of models are used: A hydraulic model, a biological model, a habitat
model and a water quality model.

The hydraulic model is developed by measuring four critical
fish habitat variables (water depth, water velocity, cover and substrate composition) at stream transects (cross
sections), at various flows. This data is used to develop a computer model that can describe how these variables change
with any flow the stream might experience.

The biological model is developed by observing fish in the
wild and describing which depths, water velocities, cover types (such as root wads, large boulders, undercut banks or
vegetation) and substrate types are most suitable for each species and life stage (spawning, fry, juvenile and adult).
Snorkeling is often used to document the locations the fish prefer.

The hydraulic model and the biological model are then
combined to form the habitat model, which describes how habitat-as defined by depth, velocity, substrate, and
cover-changes with flow for each species and life stage.

Figure 2 shows the result of such a habitat model developed
from a study we conducted on Tulpehocken Creek immediately downstream of Blue Marsh Dam, Berks County.

Finally, water quality models can be used to describe how
water quality variables, such as temperature, change with flow. For example, a bottom release of 2 million gallons per
day (mgd) of cold water from a reservoir might cool a particular stream to a temperature suitable for trout for perhaps
2 miles. A water quality model can be used to answer the question of what happens when the release is reduced to perhaps
1 mgd or increased to 3 mgd.

Commission's role

The primary authority in Pennsylvania for regulating water
withdrawals from surface waters for purposes such as municipal drinking water supplies, power plant cooling, golf course
irrigation, and snow-making lies with either the Pennsylvania Department of Environmental Protection (DEP) and/or two
interstate commissions-the Susquehanna River Basin Commission and the Delaware River Basin Commission. The Fish & Boat
Commission acts as a consultant to these agencies on matters related to stream flow protection.

Fish & Boat Commission staff reviews water withdrawal
permit applications submitted to these agencies. We then recommend stream flow protection levels or other conditions for
the permits with the objective of protecting aquatic life.

A recently completed cooperative study that involved the
Fish & Boat Commission as well as DEP, the Susquehanna River Basin Commission, the U.S. Army Corps of Engineers, the
Maryland Department of Natural Resources, and the Chesapeake Bay Foundation resulted in the development of an instream
flow computer model specific to Pennsylvania's trout streams. This million-dollar statewide study involved the
development of hydraulic models at over 100 sites distributed among 72 streams in the Appalachian Plateau and Ridge and
Valley physiographic provinces of Pennsylvania and the Piedmont province in Pennsylvania and Maryland. The computer
model that was developed can be used to predict the effects of withdrawals by a municipality, industry or other entity
on trout habitat before the withdrawal is begun. Conditions can be placed on water withdrawal permits using the
information gleaned from the model to prevent degradation of instream habitat while still often permitting some level of
water withdrawal.

Recently, the Fish & Boat Commission has been involved in
studies to evaluate the insidious but very difficult to quantify effects of groundwater pumping on stream flows. Base
flows on streams are largely supplied from groundwater aquifers. When necessary, well permits are now conditioned to
protect these flows.

Another way the Fish & Boat Commission works to protect the
natural flow regime of streams and rivers is through the review of hydroelectric license applications. The hydroelectric
industry in the United States is regulated by the Federal Energy Regulatory Commission (FERC). We work with FERC as well
as with electric utilities, private power producers, the public, local governments, non-governmental organizations such
as Trout Unlimited, and other agencies such as DEP, the U.S. Fish and Wildlife Service, and the U.S. Army Corps of
Engineers in the license review process.

We are currently working on the relicensing of both the
Wallenpaupack and Piney hydroelectric projects. The Wallenpaupack Project is located in Pike and Wayne counties on
Wallenpaupack Creek. The project discharges water into the Lackawaxen River. The Piney Project is located on the Clarion
River in Clarion County. The effect of peaking on aquatic life is being assessed at both of these projects. "Peaking"
means holding water back in the project reservoir at night to discharge high volumes of water during the day when energy
demands are greatest.

The goal of the Fish & Boat Commission in this process is
to mitigate the effects of the drought/flood nature of this unnatural flow regime and restore some sense of normalcy to
the flow patterns. An improved warmwater fish community should result in the Clarion River. An improved trout fishery in
the Lackawaxen River is a major goal of the Fish & Boat Commission in the Wallenpaupack relicensing. IFIM studies have
been conducted on both rivers to address these issues.

We believe we have made great strides in stream flow
protection in recent years in Pennsylvania. The studies and work described above have placed Pennsylvania on the cutting
edge of habitat protection through the process of regulating water withdrawals from streams and rivers as well as from
the ground. These advances in the science of stream flow protection are critical as we face a future with
ever-increasing demands on our precious water resources.

The next time you turn on your tap, flip on a light, or
drink a soda, remember that the water used to make these things possibly comes from a precious resource that we all
share, and that we should all respect and value. If we face future challenges to share the resource with this attitude,
we will help ensure that future generations can experience the wonders of Pennsylvania's natural heritage.

Leroy Young is Chief of the Aquatic Resources Section in
the Commission's Division of Environmental Services.